A laccase with a novel N-terminal sequence, a low molecular mass of 43 kDa smaller than those of previously reported laccases, a pH optimum of 4, and a temperature optimum at 70degreesC was isolated from fresh fruiting bodies of the mushroom Tricholoma giganteum. The activity of the enzyme rose steadily from 20 to 50degreesC, increased very slowly from 50 to 70degreesC, and fell slightly when the temperature was further increased to 80degreesC. The activity of the laccase underwent little changes over the pH range 3.0-5.0. However, the enzyme activity dwindled to nothing after exposure to 100degreesC for 10 min and when the ambient pH was 7 or above. The procedure used for purifying the enzyme included ion exchange chromatography on DEAE-cellulose, affinity chromatography on Affi-gel blue gel, ion exchange chromatography on CM-cellulose, and FPLC-gel filtration on Superdex 75. The laccase was unadsorbed on DEAE-cellulose and adsorbed on Affi-gel blue gel and CM-cellulose. It inhibited HIV-1 reverse transcriptase with an IC50 of 2.2 muM. (C) 2004 Elsevier Inc. All rights reserved.